It is well known in the aluminum alloy casting art that molten metal (“melt” for brevity) surface oxidation can result in various surface imperfections in cast ingots such as pits, vertical folds, oxide patches and the like, which may develop into cracks during casting or in later processing. A crack in an ingot or slab that propagates during subsequent rolling, for example, can lead to expensive remedial rework or scrapping of the cracked material.
The casting of alloys may be done by any number of methods known to those skilled in the art, such as for example, semi-continuous casting (direct chill casting (DC), electromagnetic casting (EMC), horizontal direct chill casting (HDC)), hot top casting, continuous casting, die casting, roll casting, and sand casting.
Continuous casting refers to the uninterrupted formation of a cast body or ingot. For example, the body or ingot may be cast on or between belts, as in belt casting. Casting may continue indefinitely if the cast body is subsequently cut into desired lengths. Alternately, the pouring operation may be started and stopped when an ingot of desired length is obtained. The latter situation is referred to as semi-continuous casting.
Each of the casting methods mentioned above has a set of its own inherent problems, but with each technique, surface imperfections can still be an issue. One mechanical means of removing surface imperfections from an aluminum alloy ingot is scalping. Scalping is the machining off of the surface layer along the sides of an ingot after it has solidified. Scalping is undesirable because of the inherent waste of energy and time and the generation of scrap alloy.
It is known in the art that the quality of a cast aluminum alloy ingot is related to the distribution of the melt, and the rate of melt flow into the mold. Melt distributor and melt filtration devices are described in the prior art, and include a “sock” of flexible glass cloth, disclosed in U.S. Pat. No. 3,111,732; a glass fiber bag marketed under the name “COMBO® bag” by Kabert Industries, Inc., Villa Park, Ill.; the “MINI® bag” also marketed by Kabert Industries, Inc.; and a “bag-in-a-bag” as disclosed in U.S. Pat. Nos. 5,207,974 and 5,255,731.
During ingot casting, turbulence, air-formed oxide, and surface waves in the melt generate oxides, which adversely affect the economics of ingot production. Surging, as a result of waves in the melt, entraps air in the melt and results in oxide formation. Some of the oxides are trapped by the solidifying butt shell and may act as initiation sites for butt cracks. The remaining oxides float out to the surface of the melt and accumulate in the mold cavity. The accumulated oxides grow in thickness and area until they are entrapped on the surface or in the subsurface of the molten ingot as casting proceeds. Patches of entrapped oxides, especially those at the surface, may cause surface imperfections that may lead to ingot cracks that require scalping.
Certain magnesium containing aluminum alloys, such as 7050 and other 7xxx alloys as well as 5xxx alloys such as 5182 and 5083, are especially prone to surface defects and cracking. It is known to add beryllium or other additives to the melt to control melt surface oxidation and to prevent magnesium loss due to oxidation. However, the use of beryllium or other additives can be very costly. For this reason, although beryllium and other additives are effective at controlling melt surface oxidation and surface defects in aluminum cast ingots, a suitable alternative approach is needed.
There remains a need for an effective alternative to the use of beryllium or other additives to substantially reduce the number of oxides present at the ingot surface so as to minimize the number of surface imperfections, such as vertical folds, pits, oxide patches and the like from forming during aluminum ingot casting. Such a method would be instrumental in substantially reducing the number of cracks that may form during casting or in later processing. Finally, the method preferably would have little or no adverse affect on alloy properties.
The primary object of the present invention is to provide a direct chill mold design for the casting of aluminum alloys that controls the flow of the melt so as to minimize the amount of oxides present at the surface of the ingot and therefore substantially reduce the occurrence of ingot surface imperfections, such as vertical folds, pits, and oxide patches.
Another object of the instant invention is to provide a direct chill mold design for the casting of aluminum alloys that reduces the occurrence of ingot cracking due to surface imperfections that are formed by oxides that are present at the ingot surface.
Another object of the instant invention is to provide a semi-continuous direct chill mold design for the casting of aluminum alloys that incorporates a continuous lubrication system.
A further object of this invention is to provide a method for casting aluminum alloys with improved surface quality without the need for adding beryllium or other additives to the alloy.
These and other objects and advantages are met or exceeded by the instant invention, and will become more fully understood and appreciated with reference to the following description.